Vaccines are used to stimulate an immune response in an individual to provide protection against and/or treatment for a particular disease. Some vaccines include an antigen to induce the immune response. Some antigens elicit a strong immune response while other antigens elicit a weak immune response. A weak immune response to an antigen can be strengthened by including an adjuvant in the vaccine. Adjuvants come in many different forms, for example, aluminum salts, oil emulsions, sterile constituents of bacteria or other pathogens, cytokines, and so forth.
Cytokines are proteins made by cells that affect the behavior of other cells, and unlike many adjuvants, can modulate specific immune responses. One such cytokine is the interleukin-21 (IL-21), which exerts actions on lymphoid and myeloid populations, as well as on epithelial cells, regulating both innate and adaptive immune responses. IL-21 has been shown to contribute to the functional differentiation of several CD4+ T cell subsets, to promote the proliferation and functional responses of CD8+ T cells, and to play a role in the development of B cell immunoglobulin responses. IL-21 is produced by CD8+ T cell populations as well as CD4+ T cell populations, including T follicular helper (TFH) cells, T helper type 17 (Th17) cells and natural killer T (NKT) cells.
Vaccines are also administered in many different ways (e.g., injection, orally, etc.) into many different tissues (e.g., intramuscular, intradermal, etc.). Not all delivery methods, however, are equal. Some delivery methods allow for greater compliance within a population of individuals while other delivery methods may affect the immunogenicity and/or safety of the vaccine. Accordingly, a need remains in the art for the development of safe and more effective adjuvants that increase antigenic responses irrespective of the identity of the antigen and route of administration.